Mary doesn't kown which one she should buy.

问题描述:

Mary doesn't kown which one she should buy.

首先明白简单句的概念,就是句子中只有一个主语(包括并列主语)和一个谓语(包括并列谓语),其他成分则是单词或短语构成.
Mary doesn't know which one to buy.